1. Field of the Invention
This invention relates generally to a method for placement of individual elements, including pixels, on a new substrate.
The subject of this invention is a method for the transfer and placement of individual elements or devices fabricated on a semiconductor substrate to predetermined locations in a new substrate of any material, shape and size, as well as in-situ electrical monitoring to make sure that all elements have been properly placed in these locations.
More particularly, the invention comprises a method by which individual elements are epitaxially lifted off a seed substrate and placed in predetermined and arbitrary circular holes, or receptors, on a new substrate.
2. Description of the Related Art.
In the prior art, the positioning of individual elements into predetermined locations in a new substrate (following the lifting these individual elements off the original seed substrate) is achieved using submersion of these elements and the new substrate into a fluid and allowing the fluid flow to transfer the elements to the receptor sites.
The liquid submersion method has three key drawbacks:
(1) the liquid submersion of the individual elements and the new substrate precludes the use of in-situ electrical monitoring for the realtime determination of the correctness of placement of the individual elements in the receptors;
(2) the liquid submersion also precludes the placement of various adhesives for the proper electrical attachment of the individual elements to the receptors; and
(3) the square-shaped elements and receptors described for this method are inadequate because the possibility is reduced that they be placed in the receptors correctly.
Another method for transferring individual elements fabricated on one substrate to different locations on a new substrate comprises transferring these elements to a stretchable membrane via the epitaxial liftoff process, followed by stretching the membrane to position and bond the circuit elements to their new locations. Following this bonding, the membrane layer is then released.
The obvious disadvantage of the stretchable membrane technique is the inability to position the individual elements to any arbitrary location on the new substrate due to the limitations of the stretchable membrane. Also this technique involves a double transfer process in which the circuit elements are first attached to the membrane before being stretched and bonded to the new substrate, thus decreasing the fabrication yield.
There is a need for a method for transferring individual elements fabricated on one substrate to different locations on a new substrate without the drawbacks and disadvantages described above. The present invention discloses such a method, and the individual elements can be transferred to any arbitrary location on the new substrate as well as checked in-situ for their proper electrical placement before the final bonding as described below.
The present invention is directed to a method used to lift individual elements off a seed substrate and to place these individual elements in receptors on the new substrate, using gravitational forces and vibrational energy.
The present invention is also directed to the technique used for realtime electrical verification of whether the placement of these elements in the receptors is correct. The realization of this concept allows transfer of electrical circuitry fabricated on an original substrate to a new substrate made of any material, and having any shape and size. One such example would be the realization of large area active matrix circuits on flexible substrates for displays as described hereinafter.
The method also allows the transfer of devices or circuits fabricated on a substrate made of one type of material to a substrate made of a different type of material. For instance, using this method, arrays of microelectromechanical switches (MEMs) operating in the radio-frequency regime together with their associated integrated electronics can be transferred to a large flexible antenna aperture.
Another example of particularly beneficial application of this invention pertains to the technology of large area active matrix displays. Millions of active matrix driver pixels can be prefabricated on a silicon substrate using conventional foundry processing, and then transferred to a large sheet of glass or plastic substrate as described in detail below. Flexible and conformal displays for automobiles are yet other examples of application of the method of this invention.
One of the main advantages of the method of this invention is the realization of large area electronics on flexible and lightweight plastic carriers (such as large area flexible active matrix displays) with the circuits or devices prefabricated on a crystalline semiconductor substrate (such as silicon). According to a competing technology, the devices and circuitry are fabricated on plastic carriers by means of deposited polycrystalline semiconductors which have inferior electrical characteristics compared to those fabricated on single crystal silicon. The proposed technology of this invention is superior to the competing technology mentioned above.
The circuit elements are epitaxially lifted off a seed substrate first. Then, using gravitational force and vibrational energy in a controlled environment, the circuit elements are allowed to slide down the new substrate raised from one end at a predetermined and preferably, at an optimum angle. The circuit elements lifted off the original seed substrate are preferably in the shape of a truncated cone for easy placement in holes in the new substrate which also have the same matching shape.
The resulting circular symmetry allows easy placement of the circuit elements in the receptors independent of their lateral orientation. The subsequently described unique concentric ring-shaped electrode pattern and its circular symmetry allows easy placement of the elements into the receptors and proper electrical contact between them, independent of their lateral orientation, when they arrive at these sites compared to the square-shaped elements and receptors known in prior art because the shape of square reduces the possibility of the correct placement of the elements in these sites.
Conductive interconnection between the individual elements and the new substrate is achieved by contact between circularly symmetric terminations on the bottom of each element and their corresponding connections prefabricated on the new substrate inside the receptors. Using these interconnections, the electrical contact between each circuit element and other circuitry prefabricated on the new substrate can be electrically verified in real time as the individual elements fall into the receptors.
In one aspect, the present invention provides a method for transferring circuit elements originally supported by an original substrate to locations on a new substrate. This method comprises steps of: providing said original substrate with a release member disposed upon a surface of said original substrate, said circuit elements being fabricated on top surface of said release member; defining individual elements about said circuit elements, said individual elements preferably having a conical frustum-shaped configuration; fabricating a first set of electrically conductive contacts on a surface of said individual elements, said first set of electrically conductive contacts being preferably concentrically disposed rings defining space therebetween; freeing said conical frustum-shaped individual elements by removing said release member; defining frustum-shaped individual element receptors in said new substrate, said receptors having a bottom surface and sloping side walls, said receptors being sized to receive said conical frustum-shaped individual elements; fabricating a second set of electrically conductive contacts on said bottom surface, said second set of electrically conductive contacts being concentrically disposed rings defining space therebetween, said second set of electrically conductive contacts arranged so as to match said first set of electrically conductive contacts when one of said frustum-shaped individual elements is received in ne of said frustum-shaped individual element receptors; applying an electrically conductive substance inside said receptors so as to cover at least the electrically conductive contacts on said bottom surface of said receptors, said electrically conductive substance exhibiting increased conductivity in a direction normal to said bottom of said receptors; raising said new substrate to an incline, with one end of said new substrate being higher than an opposite end of said new substrate; pouring said freed frustum-shaped individual elements onto the higher end of the surface of said new substrate having said receptors and shaking said new substrate so that free frustum-shaped individual elements fall or roll down the inclined new substrate and are received in said receptors and said rings of said first set of electrically conductive contacts being brought into conductive contact with corresponding rings of said second set of electrically conductive contacts by the electrically conductive substance inside said receptors; and removing unreceived and/or improperly received frustum-shaped individual elements which have fallen or rolled down to said opposite end of the inclined new substrate from said surface of said new substrate.
In another aspect, the present invention provides a method for transferring individual elements fabricated on an original substrate to arbitrary predetermined locations on a new substrate, the method comprising steps of providing said original substrate with a release member disposed upon a surface of said original substrate, said individual elements having been fabricated on top surface of said release member; forming circular individual elements surrounding said individual elements; shaping said individual elements to assume a frustum shape; fabricating a first set of electrically conductive contacts on top of a smaller circular surface of said frustum, said first set of electrically conductive contacts being concentrically disposed rings defining space therebetween; freeing said conical frustum-shaped individual elements by removing said release member; providing a new substrate having conical frustum-shaped individual elements receptors, said receptors having an inside area defined by a curved inside wall, a top circular opening on a surface of said new substrate, and an inside circular surface, said inside circular surface being defined as the bottom of said receptors, said top circular opening being disposed oppositely to said inside circular surface, said top circular opening having a diameter larger than a diameter of said inside circular surface, said receptors having a volume of said inside area larger than a volume of said conical frustum-shaped individual elements; fabricating a second set of electrically conductive contacts on said inside circular surface, said second set of electrically conductive contacts being concentrically disposed rings defining space therebetween, said second set of electrically conductive contacts arranged so as to match said first set of electrically conductive contacts; applying and partially curing an electrically conductive resin inside said receptors so as to cover only said bottom of said receptors; raising said new substrate to an incline, with one end of said new substrate being higher than an opposite end of said new substrate; pouring said freed conical frustum-shaped individual elements onto the higher end of the surface of said new substrate having said receptors followed by shaking said new substrate so that said freed conical frustum-shaped individual elements roll down the inclined new substrate and are trapped inside said receptors and said rings of said first set of electrically conductive contacts being in contact with corresponding rings of said second set of electrically conductive contacts; removing untrapped and/or improperly trapped conical frustum-shaped individual elements that have rolled down to said opposite end of the inclined new substrate from said surface of said new substrate and placing said untrapped conical frustum-shaped individual elements onto said surface of said new substrate; repeating said shaking followed by said step of removing until all said receptors are filled with said conical frustum-shaped individual elements and completing said curing of said electrically conductive resin inside said receptors.
Another aspect of the invention provides a method for monitoring and correcting following the transferring of the individual elements, the method for monitoring and correcting comprising steps of applying voltage pulse waveforms to the second set of metallization contacts; measuring a current pulse generated as a result of applying of the voltage pulse waveforms and repeating the steps of applying of the voltage pulse waveforms and of measuring of the current pulse with each receptor.